Systems for locating mobile terminals such as cellular telephones, tablets, or other devices that communicate with cellular networks have been available for some time. These systems are useful in emergency situations as well as in the surveillance of the mobile terminals. While low cost global positioning system (GPS) receivers are incorporated in many mobile terminals, GPS alone is often insufficient to provide the desired location information and requires the cooperation of the mobile terminal and/or cellular network, which can pose challenges in surveillance settings.
GPS typically relies on the mobile terminal receiving signals from at least four satellites. The satellite signals are not always available indoors or outside in areas having large buildings that block the reception of the signals. The time to provide the first fix in a GPS can be excessive. In addition, the location determined by the GPS in the phone must be transmitted to the cellular network in response to some command that caused the mobile transmitter to activate its GPS tracking system.
Hence, systems for locating mobile terminals that rely on the cellular network infrastructure and the transmissions between the mobile terminals and the cellular stations are still utilized. The GPS data is used to enhance the location information when the GPS signals are available. Unfortunately, these systems also require the assistance of the mobile network or the mobile terminal.
For example, systems that measure the distance from the cellular tower utilize a measurement of the round trip time between the base station and the mobile terminal to determine a distance from the base station. In such systems, the mobile terminal can be localized to a ring of positions within a sector of the station or a circular area around the station. These systems depend on timing measurements that are made by the base station, and hence, require the active assistance of the base station to provide the position measurements.
A second class of systems requires the mobile terminal to observe the time difference in the arrival of pilot signals in the downlink transmission emitted by neighboring base stations, and hence, determine its own position. This system requires the mobile terminal to be within range of multiple base stations. This technique, like that of GPS, requires the mobile transmitter to make the measurement and report it via the base station. Such systems are difficult to implement when clandestine surveillance of the mobile terminal is desired.
A third class of systems utilizes so-called local measurement units (LMUs) that detect the uplink transmissions from the mobile terminals. The LMUs are located at various known points in the cell. While the LMUs are separate from the cellular system, these systems still require cooperation with the base stations. In these systems, the mobile terminals need to be influenced to transmit during idle periods and the independent cellular positioning system also needs to discover the serving base station so that LMUs in close proximity can be configured to detect the uplink mobile terminal transmissions. Paging is used to discover the serving base station during idle periods. The mobile terminal is persuaded to think it needs to execute a handover to another base station. To this end, it generates uplink-signaling transmissions that are used to determine the time difference arrival of the uplink transmissions at the appropriate LMUs. Here, again, the local cellular system needs to be involved to trigger the mobile transmitter to make the uplink transmissions in question.
Finally, even in systems in which the mobile terminal has a GPS receiver, the GPS receiver needs to be triggered to make a measurement and report that measurement back to the base station. Hence, the cellular system is needed to assist in the information triggering and the data collection.
As new cellular networks are deployed to improve the data bandwidth available to the mobile terminals, or make other improvements, the current positioning systems also need to be upgraded. This increases the cost and complexity of the cellular networks and requires the cooperation of the various cellular providers. Accordingly, a position location system that does not require the active participation of the cellular provider or the mobile terminal would be advantageous.